Integrated accelerometers which utilize capacitive structures are well known in the art, and are becoming more prevalent in consumer electronics such as smart phones, tablets and so forth. Such accelerometers typically comprise Micro Electromechanical Systems (MEMS) technology, wherein comb-like structures have sets of conductive fingers which are spaced apart and interleaved, with one set of fingers typically movable relative to the other. During acceleration or deceleration, the conductive fingers of said structures move closer or further from each other, with an associated change in measured voltages. From the change in measured voltage, signal processing circuitry, typically within the integrated accelerometer, calculates the acceleration force applied. In typical prior art accelerometers, a comb-like structure is utilized for each orthogonal direction in a three dimensional coordinate system. The capacitive comb is just one possible MEMS implementation used by the prior art, with many alternatives known for using MEMS to obtain capacitive readout due the displacement of a proof mass. For example, it is known that a z-axis accelerometer may be realized by using the parallel plate capacitance between two plates which move relative to one another.
FIG. 1 illustrates circuitry for some prior art accelerometers in a block diagram schematic. Block 1.1 represent the comb-like structures, with C1 the capacitance between a set of fingers 1.2 and a movable set 1.3, and C2 the capacitance between another set of fingers 1.4 and the movable set of fingers 1.3. When the accelerometer moves, C1 and C2 changes accordingly, with C1 which typically increases and C2 which decreases for movement along one axis, and vice versa. The series string of C1 and C2 is typically connected between a pulsed positive supply rail 1.5 and a correspondingly pulsed negative rail 1.6, with the voltage at the centre of the series string fed via an interconnection 1.10 to a differential amplifier block 1.7, also powered from a positive and a negative rail. The voltage at 1.10 represents the delta or change in capacitances C1 and C2. If no movement occurs, and C1 and C2 do not change from their calibrated values, the voltage on 1.10 stays at zero. Any change in C1 and C2 should result in a voltage above or below zero. For instance, if C2 decreases, and C1 increases accordingly, more voltage will fall over C2, resulting in a voltage above zero at 1.10. Conversely, for an increase in C2, the voltage at 1.10 should change to below zero.
Amplifier block 1.7 feeds an amplified value of the voltage at 1.10 to an analog-to-digital (A/D) block 1.8 via interconnect 1.11, with block 1.8 also supplied from positive and negative rails. A digital value of the voltage at 1.11 is communicated by the A/D via a serial or parallel connection 1.12 to a digital signal processor (DSP) 1.9, which provides an indication of calculated acceleration, or values proportional to it, at 1.13. Typically, DSP 1.9 may be supplied with a single positive rail and ground.
One of the drawbacks of some prior art accelerometers is the requirement for both a positive and negative supply rail, as this requires additional circuitry and semiconductor real estate. Another drawback is the complexity of the circuitry, with an amplification stage, a digital conversion stage and a signal processing stage, wherein these stages typically conserve a fair amount of power. In addition, the prior art does not make direct use of the underlying mechanism present in said comb-like structures. When said conductive fingers move relative to one another, the capacitance of the structures and the relative amounts of stored charge changes. It may be advantageous to directly measure the differential change in stored charge and capacitance with a charge transfer technique which have low relative power consumption.
Another challenge with prior art MEMS solutions is the parasitic capacitances realized with practical MEMS structures, which may be on the order of one to a few pico Farad (pF), (1 pF=10−12 F). These parasitic capacitances may negatively influence the resolution and range of capacitance measurements made during acceleration detection. Prior art capacitive MEMS solutions also suffer from a large offset in the capacitance signal being measured. During production of such MEMS structures, utmost care needs to be taken to ensure that the structures are completely symmetrical, i.e. that C1 and C2 have the same value. However, in reality, due to manufacturing tolerances, C1 and C2 often differ by a value on the order of 100 femto Farad (fF), (1 fF=10−15 F), where C1 and C2 themselves have values in the range of 1 pF to 1.5 pF. Given that the typical change in capacitance due to acceleration which has to be measured is a few atto Farad (aF), (1 aF=10−18 F), such a large offset makes the measurement, often performed with an op-amp circuit, very challenging, adding complexity and cost to prior art solutions.
In addition, the two integrated circuits (IC's) respectively containing the MEMS structure and the Application Specific Integrated Circuit (ASIC) used for signal processing have to be developed in conjunction, to ensure a high level of matching and minimum offset in the differential capacitance signal. In prior art solutions, it is considered impractical to develop the MEMS structure integrated circuit in isolation, and to connect it to a signal processing ASIC not specifically tailored to it, due to said large offset in differential capacitances.
Given the above, prior art capacitive structure MEMS accelerometers may be improved by effective compensation for parasitic capacitances of the MEMS structure, as well as differential capacitance offset compensation with a large range, easing production and integrated circuit matching requirements.